Color is perceived by the human eye according to three attributes: lightness, chroma, and hue. Lightness roughly relates to the amount of light coming from the observed object (by emission or reflection), and is a quantitative parameter with a value ranging from 0 to 100. Chroma, also referred to as color saturation, relates roughly to how pure the color is (i.e., higher chroma values are less gray; gray has a chroma value of zero, and is also a quantitative parameter with a value ranging from 0 to 127. Hue is the qualitative characteristic of color that is usually referred to by name (green, red, purple, etc.), and ranges from a value of 0° (magenta) through 90° (yellow), 240° (cyan) and finally returns to 360° (magenta).
Traditional printing processes use four process colors (cyan, magenta, yellow and black; C, M, Y and K) to replicate an image. Hues other than C, M, and Y are produced by combining inks. For example, orange (O) is produced by combining M and Y, green (G) is provided by combining Y and C, and blue (B) results from combining C and M. However, since CMYK is a subtractive color system, arriving at orange, green, and blue by using two pigments instead of one tends to increase the grayness of the resultant color, i.e., it limits chroma.
Therefore, a CMYK color set has a rather limited “gamut.” The gamut of a given set of inks is the locus of points in the three-dimensional lightness-chroma-hue space that can be reproduced by that ink set. To address the limitations of CMYK as to the chroma that can be produced at any given hue, processes using more than four inks have been developed to increase the color gamut. Typically, separate orange, green and blue inks are added. Such colors are referred to herein as “redundant” colors because they provide a hue that is already obtainable by combining two primary colors (C, M, Y). However, their use confers certain significant advantages.
By using inks of these colors, it is possible to rely less upon the use of combinations of two ink pigments (which limits available chroma) to produce a given hue. Thus, including these redundant process colors along with the “primary” (CMYK) colors expands the gamut in the sense that it allows the production of higher chroma in those images areas in which they are used to partially replace two primary colors. Any (and typically, all) of these three additional colors may be used, and still further ones may be added without any theoretical limit.
To reproduce and print an image on a printing press, the image is typically provided in digital format by an image capture device (e.g., from a digital camera or scanned by a color scanner), and software then converts the image into a map that specifies how much of each of the process colors (C, M, Y and K) to apply to the printed page. Methods for specifying an appropriate amount of each of the process colors to faithfully reproduce the original color, as that color would be measured by an objective evaluation (such as by a calorimeter or spectrophotometer measuring CIE LAB values, or the like), are known in the art. Such methods are available for CMYK and CMYKOGB color sets, and they can faithfully reproduce the original color as it would be measured by an objective evaluation method (such as by a calorimeter or spectrophotometer measuring CIE LAB values or the like). Printed single-color samples produced by such methods are found by a majority of human evaluators to provide a very good match for the original colors. Of course, this requires that the evaluators make a direct visual comparison between the original scene or transparency and the colors in the printed reproduction.
Typically, the gamut of colors than can be achieved on a printing press is smaller than the gamut of colors in the original scene or color transparency. The process of mapping color from a larger color gamut (e.g., an original scene) to a smaller color gamut (printing press) is knows as “gamut compression.” Gamut compression, by definition, results in different colors in the final print than in the original scene. While less gamut compression is required for a CMYKOGB color set than a CMYK color set, some compression is usually inevitable. However, in most cases it is desirable to avoid or reduce gamut compression as much as possible.
Typical methods for compressing gamut involve reducing chroma in an amount that is in some way proportional to the chroma of the original scene or transparency: High chroma colors receive a larger amount of chroma reduction, low chroma colors receive a smaller amount of chroma reduction. Hence, the chroma value in the reproduction produced according to such mathematically proportional approaches might be called the “proportional chroma.” Numerous mathematical models exist which attempt to objectively define this relationship based solely on the color values (LAB, LCH or other) of the two color spaces. However, printed images of realistic scenes (i.e., photographs) produced according to (such) mathematically objective methods are not always found by a majority of human observers to be most pleasing. Rather, research has shown that a majority of evaluators will prefer certain subjects (e.g., green grasses, or blues skies) to be reproduced with a chroma level higher in comparison to other subjects in the original scene (or higher than their “proportional chroma”). Research has also shown that a majority of evaluators will prefer other subjects (e.g., human skin tones) to be reproduced with a chroma level lower in comparison to other subjects in the original scene (or lower than their “proportional chroma”). Even in the case where no gamut compression is required, the tendency of observers to prefer subjects like green grass, blue sky, and many “synthetic” subjects (i.e., synthetically painted, dyed, etc. subjects) to have higher chroma than the original scene has been documented.
The main point is that color preferences of pictorial images are not related solely to the mathematical attributes of colors in the images, but also to the subject matter. These preferences are thought to be related to psychology and a human's experience with specific subjects. A human fleshtone with a chroma that is higher relative to the other colors in the scene may give the observer the impression that the person is unhealthy. The exact same mathematical color in a different subject—say a “peach colored” sweater—can be reproduced (and may be preferred) at a chroma even higher than the proportional chroma. For all of these reasons, it would be useful for a user to be able to specify how much gamut compression to provide in different color areas of an image according to the subject matter, not just according to mathematical color values.